Intra-event and inter-seasonal behaviour of suspended sediment in flash floods of the semi-arid northern Negev, Israel

A better understanding of the relation between suspended sediment concentration and water discharge is gained by recognizing, amongst other things, the different impacts of two types of rainfall event: wandering, discrete, convective or convectively-enhanced, high intensity storms; and ubiquitous, multi-celled, low intensity, frontal storms. In the Levant, the first type is characteristic of autumn and spring and the flushing of sediment by the instant onset of high intensity rainfall produces a clockwise hysteresis in the suspended sediment-water discharge relation. The second type is characteristic of winter and can be marked by either anticlockwise or no hysteresis in the suspended sediment-water discharge relation; sedimentary flushing is rarely apparent. Suspended sediment concentrations in flash floods resulting from convective storms are high (typically 10⁴-10⁵ mg l^{- 1}) but correlation with water discharge is low (r = 0.57). In contrast, frontal storm runoff carries lower suspended sediment concentrations (typically 10³-10⁴ mg l^{- 1}) and correlation with water discharge is better (r = 0.78). This division assists in interpreting the high degree of scatter evident in the overall suspended sediment concentration-water discharge relation, but it reveals that much is yet to be learned about the interplay of the disposition of cellular storm tracks and sediment source areas.